Mark MetricsSubSampler and InsecureRandomGenerator functions const

It's valid to record histograms in const contexts, but
MetricsSubSampler::ShouldSample wasn't const, making it difficult to
subsample such histograms.

This CL marks ShouldSample and the functions it transitively calls as
const, and declares InsecureRandomGenerator's internal state as mutable
to enable this.

Bug: None
Change-Id: If662dc6db3088739c886690027bee3c35c255098
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6183142
Commit-Queue: Anthony Vallée-Dubois <anthonyvd@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/main@{#1409085}